The desire to maintain a youthful appearance by reducing wrinkles in the skin is an important issue in human society. Many techniques have been designed to achieve this goal. One of the techniques is, for example, skin rejuvenation, particularly methods that involve denaturation of collagen, such as thermal denaturation of collagen. Although some therapeutic applications are possible, the main area of interest is the cosmetic, or non-therapeutic, field. The efficacy of such methods in the skin depends on several factors, such as thermal and mechanical load on the tissue, age of the person undergoing the treatment, anatomical distribution of collagen inside the skin, diseases of the skin, environmental exposure, skin type, etc. At present the parameters of denaturation-based skin rejuvenation treatments are often based on trial and error.
An increasing number of these skin treatments and skin treatment systems are intended for use by consumers rather than medical professionals. These treatment systems are non-invasive—they create an effect beneath the surface of the skin without having to physically penetrate the epidermis. However, such home-use systems raise new concerns, such as concerns relating to safety and treatment efficacy. This is particularly important when the light source for performing the treatment is a laser, and incorrect operation of such a laser can result in scarring or burning of the skin at locations where the laser light passes through the skin layers.
Damage to the epidermis, for example, is highly undesirable because this may lead to complications and health risks to the person being treated, as well as social downtime. If superficial lesions are created above the dermis, petechiae (micro-bleeding) may occur due to micro-rupturing of capillaries, resulting in reduced efficacy and an increase in side effects. The formation of new collagen for the purpose of skin rejuvenation will occur if the collagen is denaturized. The efficacy of a thermal treatment for collagen denaturation, and the subsequent collagen remodeling, are high only if the temperature inside the dermis exceeds a critical temperature of 65 degrees C. At large focusing depths, the intensity in the focus may not reach the threshold required for denaturation and therefore the treatment may not produce the desired rejuvenation effects. Because the effectiveness of denaturation of collagen and the resulting skin rejuvenation depends on several factors (as indicated before), there is a need to measure the denaturation of collagen inside the skin.
Changes in the length of the collagen fibers continue during denaturation until the fibers have shrunk to a minimum length—thermal treatment beyond this phase is over-treatment because the collagen structure is completely disrupted. Over-treatment may result in unnecessary damage to surrounding tissues and other side effects without promoting any rejuvenation and tightening effect.
It is known to use the birefringence of collagen to determine the presence of collagen, and to use repeated measurements to monitor the progression of the denaturation, such as described in WO 2011/112248. Such monitoring systems may be used in combination with many different treatment methods, such as R.F. (radio frequency)-based, U.S. (ultra-sound)-based or laser-based methods. A drawback of known measurement methods is that they are relatively complex and time-consuming.
US 2011/0144503 A1 discloses a method and a system for non-invasively measuring birefringent tissues, for example collagen tissue, and changes during the treatment of such tissues, for example denaturation, by the application of RF energy. The changes are measured through linear dichroism, circular dichroism, or birefringence. The method and system optionally use polarization sensitive optical measurements to discriminate between denaturation of unidirectionally oriented strands of collagen, such as a ligament or tendon, and denaturation of planar collagen surfaces, such as the dermal layer of the skin or collagen in joint capsules.
WO 2012/135961 A1 discloses a multiphoton microscope including an excitation source for providing an optical excitation beam at an excitation wavelength λ, a scanner for scanning the excitation beam on a sample, an objective for irradiating the sample with the excitation beam scanned by the scanner and for collecting an emission beam from the sample, a first detector for detecting a plurality of multiphoton signals, and en emission light path allowing transmission from the objective to the first detector of a wavelength band limited to greater than or equal to λ/2 and less than λ, wherein the plurality of multiphoton signals have wavelengths within the wavelength band, and wherein the plurality of multiphoton signals comprise a first multiphoton signal and a second multiphoton signal of different types.
Measurement of temperature rise has the drawback that it does not provide an accurate and quantitative measure of the desired therapeutic effect, since it depends on several other factors such as age, skin optical properties and thermal properties, etc.